The present invention relates to the improved construction of bridges, roads, sidewalks, and buildings. More particularly, the present invention relates to an improved unfilled grating composite with a reinforced, prestressed or post-tensioned concrete slab. The invention also relates to a method of making an improved unfilled grating composite with a reinforced, prestressed or post-tensioned concrete slab.
The widespread deterioration of road structures, specifically bridges, has been acknowledged as a critical problem in our Nation's transportation system. The Federal Government considers hundreds of thousands of bridges structurally deficient or functionally obsolete. A major factor contributing to such classifications is a deteriorated bridge deck (the roadway surface). The life span of the bridge deck averages only one half the service life of the other components of the average bridge.
The rehabilitation and re-decking of existing deficient structures, as well as deck designs for new structures, must account for many factors affecting bridge construction and rehabilitation. These factors include increased usage, increased loading, reduced maintenance, increased use of salts for snow and ice mitigation, and the need for lower costs, lighter weight, and more efficient construction techniques.
In the mid-1980's, the first patents issued on a new grid deck designed to solve the problems of prior designs. This new grid deck is referred to as an Exodermic™ deck. An Exodermic™ deck is comprised of a reinforced concrete slab on top of, and composite with, an unfilled steel grid. This maximizes the use of the compressive strength of concrete and the tensile strength of steel. Horizontal shear transfer is developed through the partial embedment in the concrete of the top portion of the main bars The following U.S. patents all relate to various features of an Exodermic™ deck: U.S. Pat. Nos. 4,531,857, 4,531,859, 4,780,021, 4,865,486, 5,509,243, and 5,664,378. These patents all disclose unfilled grid decks composite with reinforced concrete slabs.
Historically, the Exodermic™ deck evolved from traditional concrete filled grids. The innovation of these decks was to move the concrete from within the grid to the top of the grid in order to make more efficient use of the two components. Putting the concrete on top also allowed the use of reinforcing steel in the slab to significantly increase the negative moment capacity of the design, and moved the neutral axis of the section close to the fabrication welds of the grid. A shear connecting mechanism was required between the grid and the slab to make the two components into a composite structure. This was originally provided by using a grid having main bearing bars, distribution bars, and tertiary bars. Welded to the tertiary bars were short, ½″ diameter studs which served to transfer shear and maintain a mechanical connection between components.
An Exodermic deck typically weighs 35% to 50% less than a reinforced concrete deck that would be specified for the same span. Reducing the dead load on a structure can often mean increasing the live load rating. The efficient use of materials in an Exodermic deck means the deck can be much lighter without sacrificing strength, stiffness, ride quality, or expected life.
In a revised design of an Exodermic deck, the tertiary bars were eliminated, which saved weight, cost, and fabrication problems. Thus, the grating included only main bearing bars and distribution bars. In this revised design, since there were no tertiary bars, the function of the shear transfer studs on the tertiary bars was taken over by extension of the main bars of the grid 1″ into the slab. Holes were punched in the top 1″ of the main bars, to aid in the engagement of the bars with the concrete.
In the revised design, the main bearing bars and the distribution bars are interconnected into a grating, which requires extensive fabrication. In order to assemble the grating, the main bearing bars have had fabrication holes punched into them. The distribution bars are inserted through the fabrication holes and welded to the main bearing bars at every intersection, to thereby form the grating structure.
Welding the main bearing bars and the distribution bars can induce distortion in the steel grating. Manufacturers may need to construct the steel gratings for unfilled grid decks composite with reinforced concrete slabs using purpose built jigs and a specialized welding pattern to minimize such distortion.
Because most unfilled grid decks composite with reinforced concrete slabs are constructed in environments where corrosion of the embedded and exposed steel is likely unless preventive measures are taken, the steel grating component is generally protected by hot-dip galvanizing. Warping of steel gratings due to hot-dip galvanizing is a substantial problem for all types of steel grating decks, including unfilled grid decks composite with reinforced concrete slabs. Warpage is due to a combination of stress relieving of the welds in the 850° F. molten zinc and by differential heating and cooling of the large grating panels as they are dipped in and then removed from the galvanizing kettles. This warping regularly produces grating panels that must either be reworked in the factory, or pushed and/or pulled into proper shape in the precast plant or in the field.
To eliminate some of these fabrication problems, U.S. Pat. No. 5,664,378 discloses a further variation of the Exodermic deck. This further revision eliminates not only the tertiary bars, but it also eliminates the distribution bars of the base grid component. Thus, this variation uses a base grating of only main bearing bars. It was thought that this would further reduce costs, weight, and fabrication issues. While it did achieve these objectives, it was found that eliminating the distribution bars created significant problems with shear transfer and durability.
The present invention has eliminated or minimized the problems that result when the distribution bars are eliminated. The present invention has found that using prestressed or post-tensioned concrete allows the distribution bars of the grid to be eliminated, yet still maintain effective shear transfer and durability in a grating that is made from only main bearing bars.